Testing models of fatty acid transfer and lipid synthesis in spinach leaf using in vivo oxygen-18 labeling

Abstract

Oxygen-18 labeling has been applied to the study of plant lipid biosynthesis for the first time. [13C218O2]Acetate was incubated with spinach (Spinacia oleracea) leaves and the 18O content in fatty acid methyl esters isolated from different lipid classes measured by gas chromatography-mass spectometry. Fatty acids isolated from lipids synthesized within the plastid, such as monogalactosyldiacylglycerol, show an 18O content consistent with the exogenous acetate undergoing a single activation step and with the direct utilization of acyl-acyl carrier protein by the acyl transferases of the chloroplast. In contrast, fatty acids isolated from lipids assembled in the cytosol, such as phosphatidylcholine, show a 50% reduction in the 18O content. This is indicative of export of the fatty acyl groups from the plastid via a free carboxylate anion, and is consistent with the acyl-acyl carrier protein thioesterase:acyl-coenzyme A (CoA) synthetase mediated export mechanism. If this were not the case and the acyl group was transferred directly from acyl-acyl carrier protein to an acyl acceptor on the cytosolic side, there would be either complete retention of 18O or, less likely, complete loss of 18O, but not a 50% loss of 18O. Thus, existing models for fatty acid transfer from the plastid and for spatially separate synthesis of 'prokaryotic' and 'eukaryotic' lipids have both been confirmed.